A unique key is a key that is constrained so that no two of its values are equal. The columns of a unique key cannot contain null values. The constraint is enforced by the database manager during the execution of any operation that changes data values, such as INSERT or UPDATE. The mechanism used to enforce the constraint is called a unique index. Thus, every unique key is a key of a unique index. Such an index is also said to have the UNIQUE attribute. See Unique Constraints for a more detailed description.
A primary key is a special case of a unique key. A table cannot have more than one primary key. See Unique Keys for a more detailed description.
A foreign key is a key that is specified in the definition of a referential constraint. See Referential Constraints for a more detailed description.
A partitioning key is a key that is part of the definition of a table in a partitioned database. The partitioning key is used to determine the partition on which the row of data is stored. If a partitioning key is defined, unique keys and primary keys must include the same columns as the partitioning key (they may have more columns).